This invention relates to audio signal processing and, in particular, to a circuit that uses an improved estimate of background noise for generating comfort noise.
As used herein, “telephone” is a generic term for a communication device that utilizes, directly or indirectly, a dial tone from a licensed service provider. As such, “telephone” includes desk telephones (see FIG. 1), cordless telephones (see FIG. 2), speaker phones (see FIG. 3), hands free kits (see FIG. 4), and cellular telephones (see FIG. 5), among others. For the sake of simplicity, the invention is described in the context of telephones but has broader utility; e.g. communication devices that do not utilize a dial tone, such as radio frequency transceivers or intercoms.
There are many sources of noise in a telephone system. Some noise is acoustic in origin while the source of other noise is electronic, the telephone network, for example. As used herein, “noise” refers to any unwanted sound, whether or not the unwanted sound is periodic, purely random, or somewhere in-between. As such, noise includes background music, voices of people other than the desired speaker, tire noise, wind noise, and so on. Automobiles can be especially noisy environments.
As broadly defined, noise could include an echo of the speaker's voice. However, echo cancellation is separately treated in a telephone system and involves modeling the transfer characteristic of a signal path. Moreover, the model is changed or adapted over time as the characteristics, e.g. frequency response and delay or phase shift, of the path change.
A state of the art adaptive echo canceling algorithm alone is not sufficient to cancel an echo completely. A modeling error introduced by the echo canceler will result in a residual echo after the echo cancellation process. This residual echo is annoying to a listener. Residual echo is a problem whether or not there is background noise. Even if the background noise level is greater than the residual echo, the residual echo is annoying because, as the residual echo comes and goes, it is more perceptible to the listener. In most cases, the spectral properties of the residual echo are different from the background noise, making it even more perceptible.
Various techniques, such as residual echo suppresser and non-linear processor, are employed to eliminate the residual echo. Even though a residual echo suppresser works well in a noise free environment, some additional signal processing is needed to make this technique work in a noisy environment. In a noisy environment, the non-linear processing of the residual echo suppresser produces what is known as noise pumping. When the residual echo is suppressed, the additive background noise is also suppressed, resulting in noise pumping. To reduce the annoying effects of noise pumping, comfort noise, matched to the background noise, is inserted when the echo suppresser is activated.
Those of skill in the art recognize that, once an analog signal is converted to digital form, all subsequent operations can take place in one or more suitably programmed microprocessors. Use of the word “signal”, for example, does not necessarily mean either an analog signal or a digital signal. Data in memory, even a single bit, can be a signal.
“Efficiency” in a programming sense is the number of instructions required to perform a function. Few instructions are better or more efficient than many instructions. In languages other than machine (assembly) language, a line of code may involve hundreds of instructions. As used herein, “efficiency” relates to machine language instructions, not lines of code, because the number of instructions that can be executed per unit time determines how long it takes to perform an operation or to perform some function.
In the prior art, estimating noise power is computationally intensive, requiring either rapid calculation or sufficient time to complete a calculation. Rapid calculation requires high clock rates and more electrical power than desired, particularly in battery operated devices. Taking too much time for a calculation can lead to errors because the input signal has changed significantly during calculation.
In view of the foregoing, it is therefore an object of the invention to provide a more efficient system for generating high resolution comfort noise based upon an improved background noise estimator.
Another object of the invention is to provide an efficient system for generating comfort noise that is spectrally matched to background noise.
A further object of the invention is to provide a comfort noise generator that substantially eliminates noise pumping.